The present invention relates to a method of adjusting the resonant frequency of a resonator having a plurality of coupled vibrational modes i.e., a so-called coupling resonator. There are many consumer products requiring resonators having excellent temperature characteristics and which use an AT cut quartz resonator. Recently, however, there has been a trend toward miniaturization of the various consumer products which in turn requires use of a miniaturized AT cut quartz resonator. In a resonator of this type the resonator exhibits a great deal of spurious vibrations and miniaturization thereof is difficult.
In particular, in the case of using the AT cut quartz resonator as a resonator for wrist watches, miniaturization is required and the resonator size is not sufficient as compared with that of a flexural quartz resonator of the tuning fork type.
Recently, therefore, a method of forming a resonator by means of a photographic process which is applied to IC (integrated circuit) production has been applied to resonator production, and as a result, it is possible to provide extremely miniaturized resonators, e.g. a GT cut quartz resonator having excellent frequency-temperature characteristics, having extremely thin thickness of the resonator and a flexural-torsional quartz resonator (called FT quartz resonator hereinafter) which couples a torsional vibration to a flexural vibration, and miniaturization has been made possible. However, in order that these GT cut and FT cut quartz resonators obtain excellent frequency-temperature characteristics, they make use of two vibrational modes, that is to say, a coupling of a fundamental vibration and a sub-vibration. Therefore, the temperature characteristics are determined by the frequency difference of the fundamental vibration and the sub-vibration and by the intensity of each vibration. In particular, as the ratio of the intensity of each vibration of the fundamental vibration and the sub-vibration is different for different resonators, the difference of the resonant frequency is also different, and it is therefore necessary to adjust the difference of the resonant frequency for each resonator. This adjustment takes much time, contributes to a high cost, and is unsuitable in mass production and therefore, may be one of the causes that the resonator of this type is not used in more fields of application.